Fix two gateway test failures: outbox lane scoping + SSE teardown race

- outbox: delete_message/message_info now match the exact lane first
  (a flat-lane delete/lookup with thread_id=None sees only frames with
  no thread_id) and fall back to the message_id across all lanes only
  when the exact lane matches nothing. Previously lane=None meant
  'any lane' in the first pass, so a flat-lane delete also removed
  same-id frames from threads (test expected 3 removed, got 4).

- http_server: the SSE live loop skipped queued frames when stop() set
  sub.closed before the handler thread reached the loop (descheduled
  under load between the initial hello/status writes and the loop).
  The loop now drains frames queued before the close, so the
  status{restarting} teardown broadcast always reaches the client
  before EOF (test_disconnect_broadcasts_status_restarting was flaky
  ~70% under CPU load).

gateway-plugin/http_server.py

@@ -523,12 +523,25 @@ class HttpServer:
             for snap in self._adapter.todo_snapshot_frames():
                 self._write_sse(handler, "frame", None, snap.to_json())
             # 3. Live frames (cursor=None frames have no id).
-            while not sub.closed.is_set():
-                try:
-                    item = sub.q.get(timeout=SSE_HEARTBEAT_S)
-                except queue.Empty:
-                    self._write_raw(handler, ": hb\n\n")
-                    continue
+            while True:
+                if sub.closed.is_set():
+                    # stop() can land between the initial writes above and
+                    # this loop (the handler thread is descheduled under
+                    # load): drain the frames queued before the close — e.g.
+                    # the status{restarting} teardown broadcast — so the
+                    # client sees them before EOF instead of losing them to
+                    # the closed check.
+                    try:
+                        item = sub.q.get_nowait()
+                    except queue.Empty:
+                        reason = "stopped"
+                        break
+                else:
+                    try:
+                        item = sub.q.get(timeout=SSE_HEARTBEAT_S)
+                    except queue.Empty:
+                        self._write_raw(handler, ": hb\n\n")
+                        continue
                 if item is _STOP:
                     reason = "stopped"
                     break